Physics HL · Chapter 2: Forces and Newton's Laws
2.4 Friction Thresholds and Net-Force Balance
Explore how static and dynamic friction shape acceleration outcomes under changing applied force.
Estimated time: 30 minutes
Static-to-Dynamic Transition
One of the most important behavioral shifts in mechanics occurs at the threshold of motion. While the object is at rest, static friction can match the applied force up to a ceiling set by μsR. Once that ceiling is exceeded, the contact enters sliding and friction is typically modeled with the smaller dynamic value μdR.
This explains why heavy furniture can feel 'stuck' and then suddenly easier to keep moving. The force you need to start motion can be noticeably larger than the force you need to maintain motion. In model terms, the friction law changed regime.
Interpreting Net Force From Competing Horizontal Forces
Horizontal dynamics with friction is mostly bookkeeping with signs. Choose rightward as positive, assign applied and friction forces accordingly, and compute ΣFx. If ΣFx is zero, acceleration is zero. If ΣFx is positive, acceleration is rightward; if negative, leftward.
Changing mass affects normal force, which changes friction ceilings. That means the same applied force can produce equilibrium for one mass and acceleration for another. This coupling is central in lab design and in exam reasoning about comparative scenarios.
Simulation: High-Mass, High-Threshold Setup
Start with a heavier block and moderate friction to build a large static-friction ceiling. Probe how much applied force is needed before motion appears.
Net Force Lab
Free-body visualization
Max static friction
35.32 N
Net force
0.00 N
Acceleration
0.000 m/s²
State
Static equilibrium
The free-body diagram shows static friction matching the applied pull exactly, so the net-force vector collapses and the block remains at rest.
Interpretation target: this setup should remain in equilibrium because the friction ceiling μmg is about 35.3 N. Increase force slightly above that value and the net-force arrow should become non-zero immediately. This mirrors the textbook friction graph: static friction tracks the applied force until its ceiling, then cannot keep up.
Simulation: Net Force Lab
Adjust mass, applied force, and friction coefficient to observe static equilibrium, breakaway threshold, and resulting acceleration.
Net Force Lab
Free-body visualization
Max static friction
17.17 N
Net force
2.83 N
Acceleration
0.566 m/s²
State
Sliding
The applied-force vector exceeds static-friction capacity, so a non-zero net-force vector appears and the block accelerates in that direction.
Interpretation target: once breakaway occurs, friction is no longer matching the applied force exactly, so the net force stays positive and acceleration grows. Compare this with the pre-breakaway phase where friction exactly cancels the pull and acceleration is zero.
Simulation: Reverse Pull and Sign Discipline
Flip the applied force direction to test leftward acceleration conventions and verify that friction always opposes impending or actual motion.
Net Force Lab
Free-body visualization
Max static friction
7.36 N
Net force
-20.64 N
Acceleration
-6.881 m/s²
State
Sliding
The applied-force vector exceeds static-friction capacity, so a non-zero net-force vector appears and the block accelerates in that direction.
Interpretation target: if left is negative in your axis choice, both net force and acceleration should also be negative in this run. This is a deliberate sign-check drill: same physics, different direction, identical logic.
Test Yourself
In the lab, you increase applied force gradually while keeping m and μ fixed. Which event marks the instant motion begins?
Test Yourself
For m = 10 kg and μs = 0.30 on a horizontal surface, estimate the minimum applied force needed to start motion.
Hint: Use F_threshold = μs mg with g = 9.81 m/s².